What should I do with my battery to have it live the longest?
I was told the few first times I should let it completely drain and then charge it. Should i leave it completely drain for a few days? Should I have it completely empty or completely full more often than not? Etc., etc.
A good reference for this is Battery University. They also have a helpful chart that compares each type of battery and how to treat it.
Basically, there are two ways you need to treat a battery depending on what type it is.
If it is lithium-ion, it really doesn't matter so much how you charge it. It copes very well with shallow charging and discharging, and never develops memory. The only downside to shallow discharges is that the charge meter may become inaccurate (without affecting actual battery life), but a full discharge followed by a full charge will recalibrate it. Fully discharging the battery on a regular basis may actually be more damaging to it.
If the battery is NiCad or NiMH, then it prefers to be fully discharged and then fully charged. If the battery isn't fully charged or discharged, it may develop memory and have less capacity.
Universal advice is "don't cook the battery, don't prematurely wear it out by mindlessly charging and discharging"
Full charge-discharge cycles create more wear than partial ones.
Storing some types of battery in their flattened state can be very damaging (particularly lead-acid batteries, but also some types of lithium batteries). You end up with a hardened layer of spent chemical on one or the other electrode inside the cells of the battery.
Reasonable quality information can be found here.
I'm assuming this is for a laptop. Newer laptops have better quality charging systems than older ones. More expensive laptops can have better battery management than cheaper ones, but won't necessarily.
For laptop batteries, Dell rates batteries by the number of charge / discharge cycles. A charge / discharge cycle can occur when docked as a laptop starts up and shuts down for example, despite the cycle being very short. Dell may issue battery replacement warnings after 400-600 charge-discharge cycles.
A tip to improve battery lifetime is to remove the battery when on AC power if your brand of laptop allows it. The tradeoff is that you become susceptible to your work being lost if the power fails (if not plugged into a UPS).
This all depends on the type of material the battery is made out of. Each battery material has different ideal characteristics. Look on your battery label and determine the type of battery it is. (i.e., lithium-ion (Li-ion), nickel–metal hydride (NiMH), nickel–cadmium (NiCad), etc.. Once you determine which type of battery it is, simply google the material and read about the capabilities. This should tell you all and more you would like to know.
A note on charging... Not all charging circuits are created equal, so if you have a deficient charging circuit, you might indeed want to unplug the battery after it has been fully charged, like was suggested above.
